It is will known to clean aluminum surfaces by contact with aqueous alkaline cleaning solutions. If the solution is free of ingredients that will inhibit alkaline attack on aluminum the pH of the cleaners is generally from 9 to 10, while if the solution contains ingredients that will inhibit alkaline attack of aluminum the pH may be 12 or 13. Examples of materials that will inhibit alkaline attack on aluminum are sodium phosphate and sodium silicate. It is known to include complexing agents or similar materials in alkaline cleaners.
It is often necessary to form a coating on a clean aluminum surface, for instance that has been cleaned by treatment with one of these cleaners, and traditional coating compositons for aluminum include solutions containing hexavalent chromium and fluoride, usually with additions of accelerators such as ferricyanides or molybdates. Although these solutions form coatings which provide a good base for paint they do suffer from severe environmental problems in that both fluoride and hexavalent chromium are undesirable on health and safety grounds. For instance effluent containing hexavalent chromium must be treated before discharge. The solutions are acidic and so the apparatus by which they are applied must be resistant both to acid and fluoride.
Alkaline processes for forming coatings are known but they involve the application of hot solutions containing hexavalent chromium and so still present environmental problems.
More recently processes for forming coatings without hexavalent chromium have been developed. These therefore avoid the toxic hazards of hexavalent chromium but they are generally still acidic and still contain fluorides and so still create environmental problems and the need for apparatus which is resistant to acid and fluoride. Such solutions contain, for instance, tannin, titanium or zirconium, and fluoride.
It is, therefore, an object of the present invention to provide a process for forming a coating on aluminum that is a satisfactory base for paint and which does not involve the disadvantages of the use of hexavalent chromium, fluoride, or acidic solutions.
This and other objects will become apparent to those skilled in the art from the description of the invention which follows.